Solid propellant containing fuel-oxidizer component prepared from fused oxidizers

ABSTRACT

A novel method for making a solid propellant fuel-oxidizer comprising heating a mixture of at least two solid inorganic oxidizers to the melting point of said mixture, mixing into the molten mixture a solid particulate propellant fuel, cooling the just formed oxidizer-fuel mixture, grinding said cooled mixture and then incorporating the ground oxidizer-fuel mixture into a solid propellant binder.

United States Patent [191 Haury et al.

[ 51 Sept. 24, 1974 SOLID PROPELLANT CONTAINING FUELOXIDIZER COMPONENT PREPARED FROM FUSED OXIDIZERS [75] Inventors: Vernon E. Haury, Santa Susana;

David R. V. Golding, Malibu, both of Calif. [73] Assignee: Rockwell International Corporation, El Segundo, Calif.

[22] Filed: Aug. 19, 1965 [21] Appl. No.: 482,325

[52] US. Cl 149/17, 149/19, 149/20, 149/22, 149/36, 149/40, 264/3 R [51] Int. Cl. C061) 19/00, C06b 21/00, C06d 5/06 [58] Field of Search 149/2, 19, l8, 17, 21, 149/36, 37-44; 264/3 R [56] References Cited UNITED STATES PATENTS 2,320,971 6/1943 Lindsley 149/17 2,970,898 2/1961 Fox 149/43 X 3,106,497 10/1963 Weil 149/17 3,259,531 7/1966 Lofberg 149/17 X Primary Examiner-Benjamin R. Padgett Attorney, Agent, or Firm-Robert M. Sperry [5 7 ABSTRACT 1 Claim, No Drawings SOLID PROPELLANT CONTAINING FUEL-OXIDIZER COMPONENT PREPARED FROM FUSED OXIDIZERS This invention relates to advance in propellant technology. Particularly, the invention relates to a novel oxidizer-fuel combination for utilization in solid propellants.

Prior to the herein invention there has been concern for the isolation of reactive materials in solid propellant grains. Various physical encapsulation means have been proposed for preventing the contact of the highly reactive fuel, and sometimes even oxidizer, with the binder material in the grain, or in segregating a fuel, for example, from an oxidizer. The instant invention is concerned with this problem and with another one with which the solid propellant art is confronted, namely, some of the more desirable fuel materials have extremely low density. As a result in the particle size in which they are most feasible for incorporation in a solid propellant grain and in order to incorporate sufficient quantities stoichiometrically, it is not possible to get the quantity of fuel into the grain such that the binder utilized would sufficiently hold the material in place. In other words, not enough binder would be present to cover the large surface area of the exposed fuel.

Beryllium hydride is of special interest because of the theoretically high specific impulse to be derived from its utilization as a fuel. Beryllium hydride is representative of the type of fuels referred to in that it has a low density and a high specific surface. The theoretical density is about 0.8 grams/cc, yet the actual density of commercially available material is between 0.6 and 0.7 grams/cc. The specific surface area of the beryllium hydride is in the range of 1-5 M lgram. Thus when attempts have been made to incorporate the material by conventional means into a solid propellant, it has not been possible to utilize that amount theoretically calculated to give the best performance due to the fact that the binder proportion is not sufficient to hold the material in place. Generally, the amount of beryllium hydride most desirable to incorporate into a solid propellant formulation is in the range of 20-25 percent of the formulation, depending upon the oxidizer and binder composition accompanying the material. An ideal composition using beryllium hydride which would produce theoretically maximum results would have approximately the following composition of 20 weight percent beryllium hydride, 70 weight percent oxidizer, percent or less polymeric fuel binder. To date, using prior techniques such theoretical composition has not been attainable. Through the utilization of a liquid oxidizer in a binder in a form of double base binder, up to weight percent beryllium hydride has been processed into the propellant. Even so, this has been an extremely difficult process but even at the level accomplished a problem in addition to processing has been encountered. Poor combustion efficiency resulted at such a stoichiometry. It has been found that an excess of oxidizer is necessary to burn the beryllium hydride in order to obtain necessary efficiency. The amount of oxidizer necessary is generally expressed in terms of an oxidation ratio which is the molar ratio of oxygen or its equivalent to beryllium plus carbon. Good results have been obtained only where the oxidation ratio is greater than one. As a result the presence of even small amounts of carbon would appear undesirable in the beryllium hydride compositions. It has been found in association with the herein invention that the most important criteria regarding the presence of an oxidizer is not the overall oxidation ratio but the effective oxidation ratio in the immediate vicinity of the beryllium hydride particle. This is referred to as the local stoichiometry. Thus the problem encountered in the utilization of the beryllium hydride has been to find a method for increasing the oxidizer concentration in the neighborhood of the fuel.

Thus an object of this invention is to provide a method for utilization in solid propellant applications fuel material that has a low density and high specific surface area.

Another object of this invention is to provide a method and resultant composition of a solid propellant wherein the local stoichiometry in the neighborhood of the fuel particles is increased to a desired level.

A still further object of the invention is to provide a method of improving the process ability of solid propellant compositions utilizing material having a low density and high specific surface area.

The above and other objects of this invention are accomplished by using a solid oxidizer or mixture of oxidizers which can be fused at a temperature low enough so that the oxidizer can be mixed with the fuel without decomposition or appreciable reaction. Upon cooling, the fuel-oxidizer mixture solidifies. It can then be formulated or incorporated into a solid propellant grain through several relatively conventional processes. Because the oxidizer as a portion of the solid phase has been used to wet and cover part or all the beryllium hydride surface, the surface area which must be wet by a binder is considerably reduced. It is believed that the invention will be better understood from the following detailed description and the specific examples which follow.

Beryllium hydride is being used as an example because as indicated it has not been easy to process into solid propellant formulations, yet its theoretical properties are such that it is an extremely attractive fuel. However, whatever the fuel chosen, the oxidizer to be used will of course depend upon the fuel. Several factors then must be considered when choosing an oxidizer for a given fuel. An oxidizer, of course, above all else must be compatible with the fuel chosen. For example, with beryllium hydride, strongly acidic materials are not compatible, though they might be with other chosen fuels. The oxidizer that is chosen should have a good performance potential. It must have a good oxygen or oxidizer content and a relatively favorable heat of formation. Additionally, the sensitivity to shock, heat, and the like of the oxidizer must be low enough for practical utilization. Generally, it is desirable to choose an oxidizer which will, of course, have a melting point below that of the fuel with which it is used. Additionally, the melting point must, of necessity, be at a temperature which is below the ignition point of the fuel-oxidizer mixture. A typical range of melting points which would meet the above requirements for most applications would be oxidizers having melting points of from 40l20C. Instead of a single oxidizer, it has been found often desirable to use a combination of two oxidizers which will fuse with each other and incorporate the fuel component. For example, an oxidizer may have a melting point above that which the fuel melts, or above the ignition point of the resultant mixture.

However, when it is mixed with a second oxidizer having a lower melting point the resulting eutectic formed between the two fusable oxidizers has a melting point lower than each, which might very well be within the desirable range sought. However, since most the oxidizers that are desired to be used do not melt within a desired range, the combination of two or more is necessitated by the performance properties of the oxidizer chosen. It is well known that when two salts are mixed the resultant melting point in most instances is lower than that of each of the salts. Thus, in order to utilize an oxidizer that melts at too high a temperature for compatibility with a metal chosen, a second oxidizer having a lower melting point is first heated or melted. To this first oxidizer is then added the second high melting one. The resultant mixture of the two salts will melt at a temperature probably lower than that of the first oxidizer, and thus would be within the range of that compatible with the fuel. Alternatively, one can mix the two oxidizers intimately and then subject the resultant mixture to heat whereby both are fused. For example, ammonium perchlorate cannot be fused by itself since its melting point is above its decomposition point. Thus, in order to utilize ammonium perchlorate one would first melt hydrazine nitrate and incorporate the unmelted ammonium perchlorate into the molten or heated hydrazine nitrate whereby fusion of the ammonium perchlorate with the hydrazine nitrate transpires. When this mixture is utilized it is preferred that the ammonium perchlorate be present in at least weight percent, with the normal range being 25-50 weight percent of the mixture. Generally it can be said that any oxidizer having a melting point of approximately 100C or above would react deleteriously with the more reactive fuels such as berryllium hydride. Thus, once again it would be desirable to utilize a lower melting oxidizer in combination with ones that melt in the range above 100C. Another example of the need for combining oxidizers is illustrated by hydrazine nitrate which melts at about 71C. It can thus be combined molten with beryllium hydride. However, the pressure exponent of propellants formulated with hydrazine nitrate as an oxidizer alone is very close to unity. Thus the resultant propellant is very pressure sensitive which is quite undesirable. It is preferred that the pressure exponent of propellants be in the neighborhood of 0.5-0.7. In order to accomplish this, hydrazine nitrate is fused with ammonium nitrate or ammonium perchlorate to form a eutectic which has a melting range of 40-45C. The resultant pressure exponents of this eutectic mixture of oxidizers is within the desirable range. Though the outstanding performance of hydrazine nitrate as an oxidizer is somewhat diminished by the presence of the ammonium perchlorate, it is still sufficiently above that of the conventional ammonium perchlorate oxidizer, if used by itself.

In the following list of oxidizers which serve by way of examples only, it is to be understood that the choice of the particular one or two to be utilized in combination with a fuel is determined by the previously described criteria. For example, the metal fuel can be used when oxidizers have melting points higher than those normally suitable for use with hydrides. Of course, mixtures of salts will have lower melting ranges as was indicated in the individual components themselves. Thus those oxidizers that are contemplated by way of example only which have the ability to fuse inelude: Hydrazine nitrate, hydrazine nitroform, ammonium nitrate, ammonium perchlorate, lithium perchlorate, lithium nitrate, hydrazine perchlorate, triaminoguanidinium perchlorate, triaminoguanidinium nitrate, and various other nitrate or perchlorate salts of alkali or alkaline earth metals or nitrogenous bases such as guanidine.

Virtually any metallic fuel conventionally used in solid propellant applications is suitable for the practice of the invention. The concept of the invention is particularly valuable, of course, when dealing with fuels of the type indicated wherein a low density and high specific surface area is possessed by the fuel. However, even a metal aluminum which is perhaps the most commonly used fuel metal additive in solid propellant applications will be enhanced by the herein invention due to increasing the proximity of the oxidizer to the metal fuel particles. Generally, the metal is in the granular form of 1-30 microns in size prior to mixing with the fused oxidizer in accord with the method of the invention. Samples of the fuels would include: Beryllium hydride, beryllium, aluminum, aluminum hydride, triaminoguanidinium azide, hydrazinium azide hydrazinate, triaminoguanidinium azide hydrazinium azide double salt, tetramethylammonium hydrotriborate, zirconium.

in performing the method of the invention, the oxidizer chosen is heated to its melting point. It is then mixed with the fuel chosen. After the mixture has been brought to ambient conditions whereupon solidification occurs, it is then ground or granulated to the desired size for incorporation into a propellant binder. [t is such that the same process is followed when two oxidizers are utilized wherein the two oxidizers are first fused, then the chosen fuel is added to the molten solution and mixed and suitably dispersed therein.

For most propellant applications today, the fused product would be ground to at least two different sizes in what is known as a bimodal blend. For example, a first portion of the fused product of the oxidizer and fuel would be ground to 200 microns size while a second portion would be ground to 20 microns. A typical trimodal blend where three different size particles are used for incorporation in a propellant binder would, for example, have particles of sizes 400, 200 and 10 microns while a tetramodal blend might have particles ground to sizes of 400, 200, 80, and 30 microns. Specific proportions and size distributions to be used in each instances are determined by procedures known in the art. in the combining of the fuel and oxidizer it is preferred that the weight ranges from 10-25 percent fuel to -95 percent oxidizer. In a instance propellant, the percentage of the fused mixture of fuel and oxidizer would range from -90 weight percent of the formulation with the binder material being the remainder.

Most of the conventional solid propellant binder materials can be utilized in the formulation of solid propellants incorporating the fused oxidizer and fuel of the invention. One of the most common binders utilized is polybutadiene having terminal groups selected from the class consisting of hydroxy, carboxy, and sulfnydryl radicals. Additionally, polyesters having the same functional terminal groups as the polybutadienes can be utilized as well as plastisols, including nitrocellulose, polyvinyl chloride, cellulose acetate, polyvinyl acetate, and ethyl cellulose. Additionally, polysulfides and the vinyl polymers such as acrylate polymers are contemplated within the scope of the invention. In addition to the binders, curatives are normally utilized to effect a cure of the propellant. For example, with carboxyterminated polybutadiene, an epoxy curative or an imine type curative is normally utilized. The selection of a given curative is within the state of the art and further description of that will not be given in detail.

To the propellant grain of the invention, additional conventionally used additives may be incorporated, for example, burning rate modifiers, plasticizing agents to aid in processing. Following are specific examples describing the formulation of the fuel oxidizer of the invention with the subsequent incorporation into a propellant binder.

EXAMPLE 1 One gram beryllium hydride was mixed with 1.5g crystalline hydrazine nitrate and 1.5g crystalline ammonium nitrate in a glass beaker. The beaker was placed in an oven at 80C and heated for 15 minutes. The mixture was stirred with a spatula and returned to the oven for an additional minutes. The mixture was removed from the oven and stirred once again to ensure a uniform dispersion of the beryllium hydride.

While the mixture was still hot and in the form of a stiff paste, it was pressed by hand into a Parr sample cup. A 35 mg. B-KNO pellet was used as an ignition aid and the sample bomb was purged with nitrogen gas before closing. The bomb was pressurized with nitrogen to 55 atm and the heat of combustion determined according to standard Parr bomb procedure. The measured heat of explosion was 2.175 kcal/gm. An accurate calculation of the theoretical heat of explosion requires a knowledge of the theoretical combustion products. Assuming that the products are BeO, CO, H O, CO N etc., a theoretical upper limit of 2.73 kcal/gm can be calculated, indicating an efficiency of 80 percent. Compared to current state-of-the-art this is an encouraging result because of the high (25 percent) concentration of Bel-l and because it is known that other combustion products such as beryllium nitride actually form in relatively efficient propellants containing smaller amounts of beryllium or beryllium hydride. In addition, conditions for combustion in the Parr bomb are known on the basis of experience to be less favorable than those in a rocket motor, presumably because of the rapid dilution of reactants and greater heat loss during reaction.

EXAMPLE 11 A mixture of 4g beryllium hydride and 9g hydrazine nitrate was mixed and heated as in the above example. The hot, uniformly mixed material was forced through a 32 mesh screen and allowed to cool before scraping off the extruded strands which accumulated on the under side of the screen. 3.75g of the HN-beryllium hydride mixture was added to a previously mixed composition consisting of 0.5g carboxy-terminated polybutadiene (CTPB) binder and 0.75g ground ammonium perchlorate. The resultant mass was worked with a spatula to a uniform consistency. The CTPB binder resin was cured with 8 ephr PIX-868 curative which is N,N',N '-tris l ,2(hexylene)-benzene-1,3,5- tricarboxamide (3M Co.). The final mixture was pressed by hand into a Parr bomb cup and tested as in the above example. Measured heat of explosion was 1.635 kcal/gm. Estimated maximum theoretical heat of explosion, subject to the uncertainties discussed in the preceding example is 2.25 kcal/gm. The efficiency of 73 percent is also considered an advance in the state of the art because the theoretical adiabatic flame temperature of this formulation would be expected to be significantly less than 3200K and second, because the oxidation ratio of this formulation is 0.89. Empirically, it has been found that for propellants containing beryllium or beryllium hydride, temperature greater than 3400K and oxidation ratios near 1.1 are needed for efficiency.

EXAMPLE III A mixture of 4g beryllium hydride and 15g HN was mixed and heated as described above. The resulting uniform mixture was allowed to cool and crushed between the platens of a Carver laboratory press to less than V8 in. dia. particle size. Nine and one-half grams of this crushed mixture was added to 0.5g binder composed of Hycar MTB resin (B. F. Goodrich Co.) with 20 phr Epon 828 which is the condensation product of epichlorohydrin and bis-pnenol A (Shell Chemical Co.), and l phr DMP-30 a reaction accelerator which is dimethylamino phenol (Rohm and Haas Chemical Co.). The resulting mass was worked with a spatula until of uniform consistency and a portion pressed into a pellet in. dia. X /2 in. long (about 5g weight) by means of a hydraulic press applying 30 tons pressure to a die. The pellet was removed from the die and allowed to cure three days at ambient temperature.

EXAMPLE IV A mixture of 51.9 gm hydrazine nitrate, 24.9 gm ammonium nitrate and 13.0 gm of beryllium metal powder of 17 micron average particle size prepared as described was crushed and reduced in particle size by grinding in a Wiley mill. Forty-four grams of the granulated fuel-oxidizer mixture was then mixed with 4 gms of a polymeric binder consisting of a mercaptoterminated polybutadiene prepolymer and toluene diisocyanate. One gram of beryllium hydride was also added to the mixture to improve ignitability of the propellant. One-half of the mixture was molded into a right circular cylinder 0.9 in. in diameter. The resulting grain was cemented into a stainless steel sleeve and mounted in a small rocket motor. Net weight of the propellant in the motor was 18.825 gm. The motor was fired at a chamber pressure of 1946 psia in a totally enclosed chamber. The gaseous and condensed combustion products were recovered and analyzed. The distribution of beryllium in the combustion products, based on the analyses, is shown in the following Table 1 together with comparable data for a propellant formulated conventionally with considerably less beryllium and tested with the same equipment at a chamber pressure of 1255 psia: 11 weight percent beryllium/76 weight percent ammonium perchlorate/1 3 weight percent carboxyterminated polybutadiene binder.

It is apparent from Table 1 below that the propellant formulated according to this invention gave more of the desired beryllium oxide and much less of the carbide. The carbide is undesirable because the heat of formation per equivalent of beryllium is only 7.6 percent of that of beryllium oxide. Heat of formation of the nitride is about 31 percent of that of the oxide.

TABLE I dizer comprising:

heating to its melting point a mixture of solid inor- BERYLLIUM DISTRIBUTION IN PROPELLANT COMBUSTION PRODUCTS Propellant of Standard ganic oxidizers wherein said mixture comprises hydrazine nitrate and ammonium nitrate, mixing in said molten oxidizers a solid particulate Example lV Be Propellant propellant fuel selected from the class consisting of Be 13 7 aluminum, aluminum hydride, beryllium, beryllium Be C 1.0 10.0 hydride, triaminoguanidinium azide, hydrazinium Egg? i? azide hydrazinate, triaminoguanidinium azide- 1360 89:3 sols hydrazinium azide double salt, tetramethyl- From ammomum perchlorate in igniter mixture, anlmonlllm l i and zlrconlum cooling said mixture of oxidizer and fuel to a solidified mass comprising from l0 to 25 weight percent Although the invention has been described and illus- 15 of Said fuel and from 75 to 95 weight Percent of trated in detail, it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of this invention being limited only by the terms of the appended claims.

We claim:

said oxidizer,

and grinding said cooled oxidizer-fuel mixture to a particle size sufficient for incorporation in a solid propellant binder, and therefrom forming a final propellant comprising from 80 to 90 weight percent of said oxidizer-fuel mixture.

l. The method of making a solid propellant fuel oxi- 

